The invention relates to a clamp-on ultrasonic flowmeter, a flow rate-measuring structure, and a ultrasonic transmitting-receiving device.
The clamp-on ultrasonic flowmeter is attached to a outer surface of a pipe in which a fluid flows, for measuring from outside of the pipe a volume of the fluid flowing inside of the pipe. The clamp-on ultrasonic flowmeters are generally classified into two types. One utilizes a difference of propagating rates, and another utilizes the Doppler effect.
In the mode utilizing a difference of propagating rates, a pair of ultrasonic waves are propagated under such condition that one ultrasonic wave is propagated downstream to cross the stream of fluid while another ultrasonic wave is propagated upstream to cross the stream of fluid. Then, the time required for propagating the downstream ultrasonic wave between the predetermined distance and the time required for propagating the upstream ultrasonic wave between the same distance are compared to determine the flow rate.
In the mode utilizing the Doppler effect, the flow rate is determined by measuring a rate of particle or babble flowing with the fluid, under assumption that the particle or babble moves at a rate equal to that of the moving fluid. The moving rate of the particle or babble can be determined, by detecting variation of ultrasonic frequency from that of ultrasonic wave applied to the moving particle or babble to that of ultrasonic wave reflected to the moving particle or babble.
A representative constitution of a known clamp-on ultrasonic flowmeter is illustrated in FIG. 11 in the form of a sectional view. The clamp-on ultrasonic flowmeter of FIG. 11 utilizes a difference of propagating rates of ultrasonic wave. The clamp-on ultrasonic flowmeter is composed of a pair of ultrasonic transmitting-receiving devices 1a, 1b. The ultrasonic transmitting-receiving device la is composed of a ultrasonic transducer 2a and a ultrasonic propagating element in the form of wedge 3a. The ultrasonic propagating element 3a has a bottom surface 4a and a slanting surface 5a extending from one edge of the bottom surface 4a at an acute angle. The ultrasonic transducer 2a is attached on the slanting surface 5a. The ultrasonic transducer 2a has an electrode (not shown) and a lead line (not shown) on the side facing the propagating element 3a and on another side. The combination of the electrode and lead line serves to apply electric voltage to the transducer 2a. In the same way, the ultrasonic transmitting-receiving device 1b is composed of a ultrasonic propagating element 3b having a slanting surface 5b on which the ultrasonic transducer 2b is attached.
Each of the ultrasonic transducers 2a, 2b transmits ultrasonic wave to the ultrasonic propagating element when an electric voltage is applied thereto, while it produces an electric voltage when it receives ultrasonic wave. Accordingly, the ultrasonic transmitting-receiving device 1a, 1b equipped with a ultrasonic transducer functions as a transmitter and a receiver. The ultrasonic transmitting-receiving devices 1a, 1b are provided on a pipe 6 in such manner that the ultrasonic waves transmitted by the devices 1a, 1b propagate across the fluid 7 which flows inside of the pipe in the direction indicated by arrow 8, that is, on the route 9 (indicated by a dotted line) in the directions indicated in FIG. 11 by arrows 9a, 9b. 
The flow rate of the fluid 7 flowing inside of the pipe 6 is determined by the following method. First, a voltage pulse is applied to the ultrasonic transducer 2a of the ultrasonic transmitting-receiving device 1a, so as to transmit a ultrasonic wave. The ultrasonic wave propagates in the ultrasonic propagating element 3a, a wall of pipe 6, fluid 7, a wall of pipe 6 on the opposite side, and ultrasonic propagating element 3b on the route indicated in FIG. 11 by the dotted line 9. Subsequently, the ultrasonic wave is received by the ultrasonic transducer 2b of the ultrasonic transmitting-receiving device 1b, to output a voltage signal. A period of time ( T1) from the time when the ultrasonic wave is transmitted by the ultrasonic transmitting-receiving device 1a to the time when the ultrasonic wave is received by the ultrasonic transmitting-receiving device 1b is detected. Subsequently, a voltage pulse is applied to the ultrasonic transducer 2b of the ultrasonic transmitting-receiving device 1b, so as to transmit a ultrasonic wave. The ultrasonic wave is then propagate on the same route, but in the opposite direction, and the ultrasonic transducer 2a of the ultrasonic transmitting-receiving device 1a receives the propagated ultrasonic wave. A period of time (T2) from the time when the ultrasonic wave is transmitted by the ultrasonic transmitting-receiving device 1b to the time when the ultrasonic wave is received by the ultrasonic transmitting-receiving device 1a is detected.
The period of time (T1) required for the propagation of ultrasonic wave from the device 1a to the device 1b along the arrow 9a differs from the period of time (T2) required for the propagation of ultrasonic wave from the device 1b to the device 1a along the arrow 9b. The period of time (T1) is shorter than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave from the device 1a to the device 1b is propagated at an increased rate by the aid of the flowing fluid, while the period of time (T2) is longer than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave is propagated from the device 1b to the device 1a against the stream of the fluid. Thus, the difference of the propagation period (T2xe2x88x92T1) is relative to the rate of movement of the flowing fluid 7. Therefore, the rate of movement of the flowing fluid is calculated from the difference of propagation period. The flow rate of the fluid 7 is then determined from the difference of propagation period and the sectional area of the inside of the pipe 6.
Thus, the clamp-on ultrasonic flowmeter is advantageous in that it can determine the flow rate with no direct contact with the flowing fluid. In order to employ the clamp-on ultrasonic flowmeter more advantageously, however, a study should be made on the clamp-on ultrasonic flowmeter for increasing the measuring sensitivity. One main point for increasing the measuring sensitivity of the clamp-on ultrasonic flowmeter resides in the improvement of directivity of ultrasonic wave transmitted by a ultrasonic transmitting-receiving device. Since the dimensions of the ultrasonic transducer used for the ultrasonic transmitting-receiving device are finite, the ultrasonic wave transmitted by the ultrasonic transducer does not form a plane wave with complete directivity but a plane wave with some diffused wave. Thus, a ultrasonic wave transmitted by the transmitting-receiving device necessarily contains a diffused ultrasonic wave portion deviated from the direction of propagation of the target ultrasonic wave (i.e., direction perpendicular to the vibrating surface of the ultrasonic transducer). The diffused ultrasonic wave portion cannot be received by the receiving device, and hence the measuring sensitivity decreases. Even if the diffused ultrasonic wave portion is received by the receiving device, the diffused ultrasonic wave portion having a phase deviated from the target ultrasonic wave produces irregular waveform in the signal wave. The production of irregular waveform also causes decrease of sensitivity.
Japanese Patent Provisional Publication H7-284198 describes directivity of ultrasonic wave transmitted by a ultrasonic transducer is improved by employing a combination of the ultrasonic transducer and a fiber-reinforced resinous material. In more detail, the publication describes that in the combined ultrasonic transducer and fiber-reinforced resinous material, a vibration in the longitudinal direction of the fiber is suppressed and therefore a vibration perpendicular to the fiber is enhanced, whereby the directivity of ultrasonic wave is improved. The publication suggests that the combined ultrasonic transducer and fiber-reinforced resinous material can be utilized in a flowmeter.
In view of the teaching given in the Japanese Patent Provisional Publication H7-284198, the present inventor manufactured a combined ultrasonic transducer 2 and fiber-reinforced resinous material 10 and further manufactured a pair of sensor-immersed ultrasonic flowmeters 1, as shown in FIG. 12. It is then confirmed that the sensor-immersed ultrasonic flowmeter 1 is effective to determine a flow rate of a fluid at a high sensitivity.
Then, the inventor manufactured a clamp-on ultrasonic flowmeter 1 by attaching the combination of ultrasonic transducer 1 and fiber-reinforced resinous material 10 to a conventional ultrasonic propagating element in the form of wedge, as shown in FIG. 13. He expected that the clamp-on ultrasonic flowmeter 1 of FIG. 13 shows an increased directivity of ultrasonic wave and an improved sensitivity. However, it was found that the sensitivity is not noticeably improved, contrary to his expectation.
Accordingly, the present invention has an object to provide a clamp-on ultrasonic flowmeter giving an improved high sensitivity.
The invention has another object to provide a flow rate-measuring structure giving an improved high sensitivity.
The invention has a further object to provide a new ultrasonic transmitting-receiving device.
The present inventor has studied further for improving the sensitivity of a clamp-on ultrasonic flowmeter utilizing fiber-reinforced resinous material, from the viewpoints of directivity and strength of ultrasonic wave transmitted by a ultrasonic transmitting-receiving device. As a result of the study, the present inventor has discovered that the ultrasonic wave having improved directivity which is produced by the combination of the ultrasonic transducer and the fiber-reinforced resinous material is reduced in its strength of ultrasonic wave and is varied in its waveform within the ultrasonic propagating element in the form of wedge.
The reduction of strength (i.e., attenuation) of ultrasonic wave and the variation of ultrasonic waveform are considered to be caused by conversion of a portion of the ultrasonic wave (i.e., highly directed longitudinal wave transmitted by the combination of ultrasonic transducer and the fiber-reinforced resinous material) into a traverse wave when the ultrasonic wave is propagated within the ultrasonic transmitting-receiving device. The generation of the traverse wave causes decrease of the normal ultrasonic wave which propagates in the direction perpendicular to the vibrating surface of the ultrasonic transducer. Further, the generated traverse wave overlaps with the normal ultrasonic wave to cause the variation of the target ultrasonic waveform.
Then, the present inventor has discovered that the ultrasonic wave transmitted by the ultrasonic transducer can be propagated within a ultrasonic propagating device, keeping its high directivity and giving almost no attenuation when the ultrasonic propagating device per se is produced by fiber-reinforced resinous material having a plurality of well aligned high modulus fibers, and that a clamp-on ultrasonic flowmeter having such constitution shows prominently high sensitivity as compared with that of the conventional clamp-on ultrasonic flowmeter.
Accordingly, the present invention resides in a clamp-on ultrasonic flowmeter comprising a pair of ultrasonic transmitting-receiving devices, each comprising a ultrasonic propagating element in the form of wedge having a bottom surface and a slanting surface extending from one edge of the bottom surface at an acute angle, and a ultrasonic transducer attached on the slanting surface, wherein the ultrasonic propagating element is composed of a plurality of sheet units in which each sheet unit is composed of plural high modulus fibers aligned in parallel in resinous material, so as to propagate ultrasonic wave emitted by the ultrasonic transducer onto the bottom surface at an angle perpendicular to the slanting surface.
The invention also resides in a flow rate-measuring structure comprising a pipe in which a fluid flows and a pair of ultrasonic transmitting-receiving devices arranged on the pipe on an outer surface thereof, each transmitting-receiving device comprising a ultrasonic propagating element in the form of wedge having a bottom surface and a slanting surface extending from one edge of the bottom surface at an acute angle, and a ultrasonic transducer provided on the slanting surface, wherein the ultrasonic propagating element is composed of a plurality of sheet units in which each sheet unit is composed of plural high modulus fibers aligned in parallel in resinous material, so as to propagate ultrasonic wave emitted by the ultrasonic transducer onto the bottom surface at an angle perpendicular to the slanting surface.
The invention further resides in a ultrasonic transmitting-receiving device comprising a ultrasonic propagating element in the form of wedge having a bottom surface and a slanting surface extending from one edge of the bottom surface at an acute angle, and a ultrasonic transducer attached on the slanting surface, wherein the ultrasonic propagating element is composed of a plurality of sheet units in which each sheet unit is composed of plural high modulus fibers aligned in parallel in resinous material, so as to propagate ultrasonic wave emitted by the ultrasonic transducer onto the bottom surface at an angle perpendicular to the slanting surface.
In the invention, the following embodiments are preferred.
(1) The sheet units of the ultrasonic propagating element are produced under the condition that the high modulus fibers in one sheet unit are arranged to make a right angle with the high modulus fibers in an adjoining sheet unit.
(2) The high modulus fiber has a tensile modulus of higher than 50 GPa.
(3) The high modulus fiber is carbon fiber.
(4) An elastic material sheet is attached to the bottom surface of the ultrasonic propagating element.
(5) The elastic material sheet has a convex surface (U-shaped surface) on the side not facing the propagating element.
(6) The elastic material sheet has a rate of sonic wave propagation in the range of 1,000 to 2,000 m/sec.
(7) The elastic material sheet is made of polyurethane gel.
(8) A pair of the ultrasonic transmitting-receiving devices are linearly arranged in a long case having one or more openings on a bottom thereof under the condition that the slanting surfaces on each of which the ultrasonic transducer is attached do not face each other.
(9) The pair of the ultrasonic transmitting-receiving devices are arranged under the condition that each of the transmitting-receiving device is able to move on the opening to change a distance between the transmitting-receiving devices.
(10) The pipe of the flow rate-measuring structure is made of metal such as iron.
(11) The pipe of the flow rate-measuring structure is made of polymeric material such as polyvinyl chloride.